In known calenders (for example, DE-C-3 216 182), a material web of paper, magnetic strip or the like passes through one or more pressing gaps which are formed in each case between two pressing rollers. Here, heatable tempering rollers, particularly in the form of a heatable pressing roller, can be provided in order to deliver heat to the material web and hence improve deformation in the pressing gap. For this purpose, by means of guide rollers it is ensured that the material web rests on the tempering roller over a predetermined circumferential angle. There are also cases in which coolable tempering rollers are used, for example in the output section of the calender in order to cool down an excessively hot paper web before winding.
If the material web is to be applied to the surface of the tempering roller, an air layer forms between the roller circumference. This air layer arises because both the material web and the roller entrain air molecules in the boundary layer and because the radially inwardly directed forces, which arise by tensile stress on the web, are partially eliminated by centrifugal forces. The higher the speed the moving material web is, the greater this effect is. The aforementioned air layer hinders heat transfer between the tempering roller and the material web.
It is an object of the invention, in a calender of the kind mentioned hereinbefore, to improve the heat transfer between the tempering roller and the material web.